1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
An image forming apparatus using an electrophotographic system has a photosensitive drum, charging means, expo sure means, a developer bearing member, transfer means, cleaning means, and fixing means. The charging means charges the surface of the photosensitive drum uniformly (charging step). The exposure means exposes the charged photosensitive drum to the light, to form an electrostatic latent image (exposure step). The developer bearing member forms a toner image on the photosensitive drum as a developer image by supplying toner as developer to the electrostatic latent image (developing step). The transfer means transfers the toner image formed on the photosensitive drum to a transfer material such as a sheet of paper (transfer step). The cleaning means removes the residual toner remaining on the photosensitive drum after the transfer of the toner image (cleaning step). The fixing means fixes the toner image onto a recording material (fixing step).
Subsequent to the cleaning step, the photosensitive drum is subjected to the charging step again. In so doing, before subjected to the charging step again, the potential of the photosensitive drum obtained prior to the charging step is made uniform by pre-exposure means (pre-exposure step) to remove the unevenness in charge potential caused by a history of exposures in the exposure step. The photosensitive drum is subjected to an electrophotographic process while repeating the foregoing steps, and used for image formation.
The toner remaining on the photosensitive drum after the transfer step is removed from the surface of the photosensitive drum by the cleaning means and becomes waste toner, although waste toner is preferably not produced from the perspective of environmental protection, effective use of resources, reducing the size of the apparatus, and the like. There has been known a cleaner-less image forming apparatus in which the developer bearing member “develops and cleans simultaneously” to remove/collect the untransferred toner which used to be collected by the cleaning means, so that the removed/collected untransferred toner can be reused (Japanese Patent Application Publication No. S59-133573). The charging means may be a corona charging apparatus, a contact charging apparatus, and the like. From the perspective of the environment and reducing the size of the apparatus, the adoption of the contact DC charging system has been promoted in recent years.
Incidentally, the image forming apparatus employing the cleaner-less system might charge some of the toner to the opposite charging polarity, and this toner charged to the opposite charging polarity might adhere to the charging member which comes into contact with the photosensitive drum. The adhesion of the toner to the charging member is likely to have an impact on the charging capacity acting on the photosensitive drum.
For the purpose of preventing the adhesion of the toner to the charging member, the surface of the charging member is cleaned when the number of pages printed exceeds a predetermined number. According to Japanese Patent No. 3030188, once the number of pages printed reaches a multiple of 100, the transfer bias is turned off as the print job completes.
When the rear edge of the print area on the photosensitive drum passes through the position of the neutralization lamp, the neutralization lamp is turned off and the charging bias is turned off when the rear edge of the print area passes through the position of the charging roller functioning as the charging member. As a result, the toner of opposite charging polarity adhering to the charging roller shifts from the charging roller to adhere to the surface of the photosensitive drum. When, in this state, the toner of opposite charging polarity passes through the developing roller functioning as the developer bearing member, the toner of opposite charging polarity changes to toner of regular polarity by being rubbed by the developing roller that is applied with a bias lower than the surface potential of the photosensitive drum by an absolute value, and then the toner of regular polarity is collected by the developing roller. In this manner, the toner of opposite charging polarity is prevented from accumulating on the charging roller.